Tackling the challenge of large model spaces

Abstract: The ab initio approach to nuclear structure allows us to describe atomic nuclei with controlled and systematically improvable approximations. Applying it to nuclei that are at the same time both heavy and open-shell is largely impossible with current many-body techniques. This is due to the computational cost of handling huge dense tensors. 

I will show how some surprisingly simple tricks may help us to tackle this hurdle. These tricks are driven by the observation that different contributions to ab initio calculations describe different physics. We leverage this by using adapted model spaces. 

In addition, we use modern linear algebra methods to develop dimensionality reduction techniques based on the singular value decomposition. By avoiding the construction of large many-body tensors in the first place, we are able to extend the reach of ab initio calculations to nuclei where standard approaches would be too expensive to run.